net: openvswitch: Annotate struct dp_meter with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct dp_meter.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: dev@openvswitch.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922172858.3822653-12-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/openvswitch/meter.h

@@ -39,7 +39,7 @@ struct dp_meter {
 	u32 max_delta_t;
 	u64 used;
 	struct ovs_flow_stats stats;
-	struct dp_meter_band bands[];
+	struct dp_meter_band bands[] __counted_by(n_bands);
 };
 
 struct dp_meter_instance {